CT-type luggage security inspection systems have become an important measure for detecting explosives, and are widely applied in public places such as airports, stations or the like. The principle of the CT detection system is to obtain projection data of the luggage by scanning, obtaining tomographic images thereof by using a reconstruction algorithm, identifying explosives according to an identification algorithm, and giving an alarm. The scanning is implemented by rotating a slip ring installed with an X-ray source and detectors while the object travels on a belt, and therefore, the scanning orbit is a spiral orbit. In order to satisfy the requirements of the security inspection authorities for the pass rate of the luggage, the speed at which the belt travels must achieve a certain value, for example, 0.3 m/s-0.5 m/s. In order to collect complete projection data required for reconstructing images, it needs to increase the rotation speed of the slip ring or increase a row number of detectors. With respect to the slip ring, in consideration of the mechanical intensity and the stability, the rotation speed cannot be increased infinitely. On the other hand, in consideration of the limit value of the cone angle of the X-ray source and the hardware cost of the detectors, the row number of detectors also cannot be increased infinitely. In overall consideration of the various factors above, the most effective manner to improve the pass rate of the luggage is to increase the scanning pitch.
In recent years, a series of results have been achieved in the field of spiral CT reconstruction algorithms, for example, the Katsevich algorithm, the PI algorithm, the CB-FBP algorithm or the like. However, all of the algorithms need to satisfy a certain pitch condition, and when the pitch and the cone angle increase, it results in projection data missing, an error in the reconstructed result increasing, or even artifacts being introduced. In order to ensure that the quality of the images satisfies requirements, the pitch factor is generally no more than 1.5.
In the security inspection CT system, real time performance of the reconstruction algorithm is also an important index. In the reconstruction algorithm, filtering in a non-one-dimensional shift-invariant form, cone beam back projection with a distance weighting factor, solution of a large number of non-linear equations, and use of a large back projection angle range will result in inefficiency of the reconstruction algorithm, and such algorithms should be avoided. Therefore, some of the above reconstruction algorithms have steps that influence the efficiency of the algorithms, and are also primarily used in case the projection data is complete or redundant.
For these systems, a problem exists in that data is missed when operating at a large pitch and thus the related art described above cannot be directly applied to a CT system with a large pitch.